Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGFigure 20-17.— Generation of sine-wave voltage.41.19right of the figure represents successive valuesof the a-c voltage induced in the conductor asit moves at uniform speed through the 2-polefield, because the instantaneous values of rotationallyinduced voltage are proportional to thesine of the angled that the rotating vector makeswith the horizontal.The sine wave in figure 20-17 represents onecomplete revolution of the armature or onevoltage cycle. The frequency of a-c voltage ismeasured in cycles per second (cps) and maybe determined by the following formula:whererpmP: frequencyf = P X rpm120(in cps; according to theNational Bureau of Standards SpecialPublication 304, frequency in cyclesper second in the International Systemsof Units is expressed as Hertz (H2).One hertz equals one cycle per second.)revolutions per minutenumber of poles in the generatorA generator made to deliver 60 cps, andhaving two field poles, would need an armaturedesigned to rotate at 3600 rpm.PROPERTIES OF A-C CIRCUITSResistance, the opposition to current flow,has the same effect in an a-c circuit as it doesin a d-c circuit. However, in the application ofOhm's law to a-c circuits, other propertiesmust be taken into consideration.Inductance is that property which opposesany change in the current flow and capacitanceis that property which opposes any change involtage. Since a-c current is constantly changingin magnitude and direction, the propertiesof inductance and capacitance are always present.The amount of opposition to current flow inan inductive circuit is referred to as its inductivereactance, Xj^. The value of inductive reactance(in ohms) depends on the inductance of the circuitand the frequency of the applied voltage. Expressedin equation form,X^ :27rfL504